Epitaxial superconducting δ-MoN films grown by a chemical solution method.

The synthesis of pure δ-MoN with desired superconducting properties usually requires extreme conditions, such as high temperature and high pressure, which hinders its fundamental studies and applications. Herein, by using a chemical solution method, epitaxial δ-MoN thin films have been grown on c-cut Al(2)O(3) substrates at a temperature lower than 900 °C and an ambient pressure. The films are phase pure and show a T(c) of 13.0 K with a sharp transition. In addition, the films show a high critical field and excellent current carrying capabilities, which further prove the superior quality of these chemically prepared epitaxial thin films.

One- and two-electron reactivity of a tantalum(V) complex with a redox-active tris(amido) ligand.

A new redox-active, tris(amido) ligand platform, bis(2-isopropylamino-4-methoxyphenylamine [NNN(cat)](3-), has been prepared and used in the preparation of tantalum(V) complexes. The ligand was prepared in its protonated form by a three-step procedure from commercially available 4-methoxy-2-nitroaniline and 1-iodo-4-methoxy-2-nitrobenzene. Direct reaction of [NNN(cat)]H(3) with TaCl(2)Me(3) afforded five-coordinate [NNN(cat)]TaCl(2) (1), which accepted the strong sigma-donor ligand (t)BuNC to form the six-coordinate adduct [NNN(cat)]TaCl(2)(CN(t)Bu) (2). Complex 1 is formally a d(0), Ta(V) complex; however, one- and two-electron reactivity is enabled at the metal center by the redox-activity of the ligand platform. Complex 1 was oxidized by one electron to afford the radical species [NNN(sq*)]TaCl(3) (3), which was characterized by solution EPR spectroscopy. Cyclic voltammetry studies of complex 3 showed clean one-electron oxidation and reduction processes at 0.148 and -0.324 V vs [Cp(2)Fe](+/0), indicating the accessibility of three oxidation states, [NNN(cat)](3-), [NNN(sq*)](2-), and [NNN(q)](-), for the metallated ligand. Complex 1 also can undergo two-electron reactions, as evidenced by the reaction with nitrene transfer reagents to form tantalum imido species. Thus 1 reacted with organic azides, RN(3) (R = Ph, p-C(6)H(4)Me, p-C(6)H(4)(t)Bu), to form [NNN(q)]TaCl(2)(NR) (4). Similarly, the tantalum diphenylmethylidenehydrazido complex, [NNN(q)]TaCl(2)(NNCPh(2)) (5), was formed by reaction of 1 with the diazoalkane, N(2)CPh(2).

Group IV imino-semiquinone complexes obtained by oxidative addition of halogens.

An isostructural series of titanium, zirconium, and hafnium complexes, M[ap] 2L 2 (M = Ti, Zr, Hf; L = THF, pyridine), of the redox-active 4,6-di- tert-butyl-2- tert-butylamidophenolate ligand ([ap] (2-)) have been prepared. The zirconium and hafnium derivatives react readily with halogen oxidants such as XeF 2, PhICl 2, and Br 2, leading to products in which one-electron oxidation of each [ap] (2-) ligand accompanies halide addition to the metal center. Iodine proved to be too weak of an oxidant to yield the corresponding oxidative addition product, and under no conditions could halogen oxidative addition products be obtained for titanium. According to X-ray crystallographic studies, the zirconium and hafnium oxidation products are best formulated as MX 2[isq.] 2 ([isq.] (-) = 4,6-di- tert-butyl-2- tert-butylimino-semiquinonate; M = Zr, Hf; X = F, Cl, Br) species, in which the molecule is symmetric with each redox-active ligand in the semiquinone oxidation state. Temperature-dependent magnetization measurements suggest a singlet ( S = 0) ground-state for the diradical complexes with a thermally accessible triplet ( S = 1) excited state. Solution electron paramagnetic resonance (EPR) spectra are consistent with this assignment, showing both Delta m s = 1 and Delta m s = 2 transitions for the antiferromagnetically coupled electrons.

Pi*-pi* bonding interactions generated by halogen oxidation of zirconium(IV) redox-active ligand complexes.

The new complex, [Zr(pda)2]n (1, pda2- = N,N'-bis(neo-pentyl)-ortho-phenylenediamide, n = 1 or 2), prepared by the reaction of 2 equiv of pdaLi2 with ZrCl4, reacts rapidly with halogen oxidants to afford the new product ZrX2(disq)2 (3, X = Cl, Br, I; disq- = N,N'-bis(neo-pentyl)-ortho-diiminosemiquinonate) in which each redox-active ligand has been oxidized by one electron. The oxidation products 3a-c have been structurally characterized and display an unusual parallel stacked arrangement of the disq- ligands in the solid state, with a separation of approximately 3 A. Density functional calculations show a bonding-type interaction between the SOMOs of the disq- ligands to form a unique HOMO while the antibonding linear combination forms a unique LUMO. This orbital configuration leads to a closed-shell-singlet ground-state electron configuration (S = 0). Temperature-dependent magnetism measurements indicate a low-lying triplet excited state at approximately 750 cm-1. In solution, 3a-c show strong disq--based absorption bands that are invariant across the halide series. Taken together these spectroscopic measurements provide experimental values for the one- and two-electron energies that characterize the pi-stacked bonding interaction between the two disq- ligands.

Catalytic reactivity of a zirconium(IV) redox-active ligand complex with 1,2-diphenylhydrazine.

Two-electron reactivity of [N2O2red]ZrL3 (1a, N2O2(red) = N,N'-bis(3,5-di-tert-butyl-2-phenoxy)-1,2-phenylenediamide, L = THF) was explored with halogens and 1,2-diphenylhydrazine. Despite a formal d0 zirconium(IV) metal center, halogen oxidative addition occurred to form [N2O2(ox)]ZrCl2(THF) (2) with two-electron oxidation of the ligand. This ligand redox activity allows catalytic reactivity with 1,2-diphenylhydrazine resulting in disproportionation to form aniline and azobenzene via a putative zirconium-imide intermediate.

Lithium-alkyl, -aryl, and -amido ligand-exchange dynamics of dianionic zirconium(IV) complexes with chelating amidophenolate ligands.

The synthesis, characterization, and solution behavior of a series of six-coordinate zirconium(IV) dianions [ZrX2(ap)2]2- (ap = 2,4-di-tert-butyl-6-(tert-butylamido)phenolate; X = Ph, 3a; X = p-tolyl, 3b; X = Me, 4; X = NMe2, 5) are described. Complexes 3-5 were prepared by treating the neutral zirconium complex Zr(ap)2(THF)2 (1) with 2 equiv of LiX or by the direct reaction of apLi2 and LiX with ZrCl4. The complexes were isolated as lithium-etherate salts, and they were characterized by NMR spectroscopy and single-crystal X-ray diffraction. In non-coordinating solvents such as benzene-d6, complexes 3-5 are robust in solution, but in coordinating solvents such as THF-d8, dissociation of LiX was observed. The rate of LiX loss was evaluated by exchange reactions; the reaction rate constants span nine orders of magnitude at 298 K, with the slowest reaction being the dissociation of PhLi from 3a (tau1/2 = 4 h) and the fastest reaction being the dissociation of LiNMe2 from 5 (tau1/2 = 53 mus). In the case of LiNMe2 dissociation from 5, activation parameters suggest that the rate-determining step is purely dissociative; however, for diphenyl and dimethyl complexes 3a and 4, respectively, activation parameters suggest a solvent-assisted rate-determining step.

"Oxidative addition" to a Zirconium(IV) redox-active ligand complex.

A strategy to enable reactivity analogous to oxidative addition is presented for d(0) transition-metal complexes. The reaction of the redox-active ligand 2,4-di-tert-butyl-6-tert-butylamidophenolate (ap) with ZrCl(4)(THF)(2) affords the new complex Zr(IV)(ap)(2)(THF)(2). This compound is formally zirconium(IV) and contains no d electrons; however, exposure of Zr(IV)(ap)(2)(THF)(2) to chlorine gas results in swift chlorine addition at the zirconium metal center via one-electron oxidation of each ap ligand. The diradical product, Zr(IV)Cl(2)(isq)(2) (isq = 2,4-di-tert-butyl-6-tert-butyliminosemiquinone), has been characterized by X-ray crystallography, electron paramagnetic resonance spectroscopy, and SQUID magnetometery.

Azide addition to give a tetra-azazirconacycle complex.

Addition of the dilithium salt, ortho-(Me3SiNLi)2C6H4, to ZrCl4 affords a base-free, D2d-symmetric complex Zr(IV)[ortho-(Me3SiN)2C6H4]2 (2), with rigorously planar ortho-phenylenediamine ligands. Lewis acidic 2 readily coordinates donor ligands such as NHEt2 to give the five-coordinate complex, Zr(IV)(NHEt2)[ortho-(Me3SiN)2C6H4]2 (3), which is also accessible by the reaction of Zr(NEt2)4 with 2 equiv of ortho-(Me3SiNH)2C6H4. Aryl azides react with 2 and 3 to give an unusual tetra-azametallacycle complex, 4, via 1,2-addition of a ligand N-Si bond to the organic azide. An X-ray crystal structure of 4 reveals a planar, five-membered metallacycle comprising the zirconium atom, one nitrogen atom of the ortho-(Me3SiN)2C6H4 ligand, and all three nitrogen atoms of the aryl azide.